They return their books to the libraryon time.此句可改为：They _ back their books _ the library on time.
问题描述：
They return their books to the libraryon time.
此句可改为：
They _ back their books _ the library on time.
答
give...to
答
They give_ back their books to_ the library on time.
give back sth to sb=return sth to sb'把某物归还给某人
答
they give back their book to the library on time.